American sea-launched test vehicle. The second phase of Sea Launch was to demonstrate the concept on a larger scale, with a rocket with a complex set of guidance and control systems. Sea Horse used one of 39 surplus Corporal missiles that Truax obtained from the Army and successfully demonstrated ignition in the ocean of a rocket stage.

Tests of the 9,000 kgf pressure fed, acid/aniline rocket were made from a barge in San Francisco Bay at Tiburon. The engine was first fired several meters above the water, then lowered and fired in successive steps until reaching a considerable depth. Firing from underwater posed no problems, and there was substantial noise attenuation. In actual operation, it was planned to tow the missile to its launch site, flood attached compartments so that the missile rode vertically in the water, and then fire it. In order to support such a program, tentative agreements were made with the Navy for Aerojet to take over an unused facility at Tiburon. All of this was done on a modest amount of IR&D funding.